This invention relates to an optical element in which a light emitting portion is disposed to a flexible polymeric optical waveguide channel and a manufacturing method thereof.
Methods of manufacturing polymeric waveguide channels proposed so far include, for example, (1) a method of impregnating a film with a monomer, selectively exposing a core portion to change the refractive index and bonding the film (selective polymerization method), (2) a method of coating a core layer and a clad layer and then forming a clad portion by use of a reactive ion etching (RIE method), (3) a method of using photolithography of conducting exposure and development by use of a UV-ray curable resin including a polymeric material with a light sensitive material added therein (direct exposure method), (4) a method of utilizing injection molding, and (5) a method of coating a core layer and a clad layer and then exposing the core portion to change the refractive index of the core portion (photo-bleaching method).
However, the selective polymerization method (1) involves a problem in bonding the film, the method (2) or (3) increases the cost because of the use of photolithography, and the method (4) has a problem in the accuracy of the diameter for the obtained core. Further, the method (5) has a problem in that no sufficient difference for the refractive index can be obtained between the core layer and the clad layer.
At present, only the methods (2) and (3) are practically excellent in view of the performance but they involve the problem of the cost as described above. Then, none of the methods (1) to (5) is suitable to the formation of the polymeric waveguide channel to a large-area, flexible plastic substrate.
Further, as a method of manufacturing a polymeric optical waveguide channel, a method of filling a polymer precursor material for core into a pattern substrate (clad) in which a pattern of grooves as capillaries is formed, then curing the same to prepare a core layer and bonding a planar substrate (clad) thereon has been known. However, since the polymer precursor material is thinly filled and cured not only in the capillary grooves but also between the pattern substrate and the planar substrate entirely to form a thin layer of a composition identical with the core layer, it involves a problem that light leaks through the thin layer.
As one method for overcoming the problems, David Hart has proposed a method of manufacturing a polymeric optical waveguide channel by securing a patterned substrate in which a pattern of grooves as capillaries is formed and a planar substrate by a clamping jig, then sealing a contact portion between the patterned substrate and the planar substrate with a resin, then decreasing the pressure and filling a monomer (diallylisophthalate) solution in the capillaries (Japanese Patent No. 3151364). This is a method of lowering the viscosity of the filling material by use of a monomer instead of use of the polymer precursor material as the core forming resin material, filling the material into the capillary by utilizing the capillary phenomenon while keeping the monomer from filling the portions other than the capillary.
However, since the method uses the monomer as the core forming material, it involves a problem that the volumic shrinkage is large when the monomer is polymerized into a polymer to increase the transmission loss of the polymeric optical waveguide channel.
Further, this is a complicated method of securing the patterned substrate and the planar substrate by clamping, or further sealing the contact portion with a resin, so that it is not suitable for mass production and, as a result, the reduction of the cost cannot be expected. Further, this method cannot be applied to the manufacture of a polymeric optical waveguide channel that uses a film in the thickness of mm order or 1 mm or less as the clad.
Geroge M. Whitesides, et al. of Harvard University have recently proposed a method of a capillary tube micromold as one of lithographic technics, as a new technology of preparing a nano-structure. This is a method of preparing a master substrate by utilizing photolithography, transferring the nano-structure of the master substrate to a polydimethylsiloxane (PDMS) template by utilizing close adhesion and easy releasability of PDMS and casting a liquid polymer into the template by utilizing the capillary phenomenon and curing the same. Detailed illustrative descriptions are contained in SCIENTIFIC AMERICAN, September 2001 (Nikkei Science; December 2001).
Further, Kim Enoch, et al. in the group of George M. Whitesides of Harvard University have filed a patent application regarding a capillary tube micromold method (U.S. Pat. No. 6,344,198).
However, even when the manufacturing method described in the patent is applied to the manufacture of the polymeric optical waveguide channel, since the cross sectional area of the core portion in the optical waveguide channel is small, it takes much time for forming the core portion and is not suitable for mass production. Further, it has a drawback that the monomer solution causes a volumic change when it is polymerized into a polymer, which changes the shape of the core to increase the transmission loss.
Further, B. Michel, et al. of IBM Zurich Research Institute have proposed a lithographic technology at high resolution using PDMS and reported that the technique can provide resolution at several tens μm. Detailed illustrative descriptions are contained in IBM J. REV. &DEV. vol. 45 No. 5, September 2001.
As described above, the soft lithographic technique or the capillary tube micromold method using PMDS is a technique which has recently attracted attention mainly in the United States as the nano-technology.
However, in the manufacture of the optical waveguide channel by use of the micromold method as described above, decrease of the volumic shrinkage during curing (accordingly, lowering of the transmission loss) and lowering of viscosity of the filled liquid (such as monomer) for easier filling cannot be compatible with each other. Accordingly, when preferences is attached to the lowering of the transmission loss, the viscosity of the filled liquid cannot be lowered below a certain limit, which retards the filling speed and mass production cannot be expected. Furthermore, the micromold method is based on the premise of using glass or silicon substrates for the substrates and use of a flexible film substrate is not taken into consideration.
By the way, in recent IC technology or LSI technology, attention has been focused on conducting optical wiring between apparatuses, between boards in an apparatus and within chips for improving the operation speed or the integration degree instead of conducting electric wiring at high density.
As the device for optical wiring, Japanese Patent Laid-Open No. 2000-39530, for example, describes an optical element for a polymeric optical waveguide channel having a core and a clad surrounding the core, which has a light emitting element and a light receiving element in the direction of laminating a core and a clad, and having an incident side mirror for incidence of light from the light emitting element to the core and an exit side mirror for emission of light from the core to the light receiving element in which a clad layer is formed in a concave shape at a portion corresponding to an optical channel from the light receiving element to the incident side mirror and from the exit side mirror to the light receiving element to converge the light from the light emitting element and the light from the exit side mirror. Further, Japanese Patent Laid-Open No. 2000-39531 describes an optical element in which light from the light emitting element enters the core end face of a polymeric optical waveguide channel having a core and a clad surrounding the core, wherein the light incident end face of the core is formed so as to provide a convex surface to the light emitting element thereby converging the light from the light emitting element to suppress the waveguide loss.
Further, Japanese Patent Laid-Open No. 2000-235127 describes a photoelectronic integrated circuit in which a polymeric optical waveguide channel is assembled directly on a hybridized photoelectronic circuit substrate in which electronic elements and light elements are integrated.
By the way, when the element described above can be incorporated, for example, by being bent into the apparatus in the optical wiring, the degree of freedom for designing the assembling of the optical wiring can be increased and, as a result, integration degree of IC or LSI can be improved.
However, since both of the optical element and the photoelectronic integrated circuit lack in the flexibility, it is impossible to incorporate the same, for example, by bending into the apparatus. Furthermore, since the optical element and the photoelectronic integrated circuit have to be used with the core end face being formed into a convex shape or use of the mirror together, this inevitably complicates structure. The reason for requiring formation of the core end face into the convex shape or converging of light by use of a lens as described above is that since the semiconductor laser element as the light emitting element used, for example, in the optical element generates a great amount of heat and the heat can no more be dissipated when the element is used merely in close adhesion with the polymeric waveguide channel to cause operation failure, it is necessary to provide a gap between the polymeric waveguide channel portion and the light emitting element to release heat while a diverging angle is present for the spot of a semiconductor laser (accordingly, light diverges as the gap is larger to bring about a difficulty in confining light in the optical waveguide channel).
Further, while both of the optical element and the photoelectronic integrated circuit include the polymeric optical waveguide channel, both are manufactured by utilizing the photolithographic method, which complicates the structure, causes a problem such as liquid wastes and results in large environmental loads.
As described above, the flexible polymeric optical waveguide channel sheet itself has not been provided at all so far and, in addition, an idea of connecting the light emitting element to the end face of the polymeric optical waveguide channel sheet thereby forming an optical element used for light optical wiring to avoid loss of flexibility has not been proposed at all.